Bug handling represents a major effort in most software projects. To improve this relevant task, software organizations must first understand the current status of their bug resolution process. Although there are plentiful research in bug reports, few of them address the bug handling workflow to better understand and reason about the maintenance process. To this purpose, we report a characterization study focused on the typical workflow followed by Mozilla Firefox developers when resolving bugs. We propose the concept of Bug Flow Graphs (BFG) to help understand the characterization. We analyze 13,564 bugs reported for Firefox in 2015 and we discovered some interesting characteristics of Firefox’s bug workflow: (a) when a bug is not formally assigned to a developer it requires ten more days to be resolved; (b) approximately 94% of duplicate bugs are closed within two days or less after they appear in the tracking system (which reveals the efficiency of Firefox’s duplicate bug detection procedures); (c) incomplete bugs, which are never assigned to developers, usually require 70 days to be closed; (d) more skilled developers show a faster resolution time than less skilled ones; (e) for less skilled developers a bug usually spends more time waiting to be assigned than being fixed.